DESCRIPTION(Adapted from applicant's abstract): Serotonin is a neurotransmitter and neuromodulator used in the control of aggression, movement, and gut motility in vertebrates and invertebrates. In the past few years, serotonin has been found to be "borrowed" rather than synthesized by some central and peripheral neurons in vertebrates. Recent results indicate that a set of 60 neurons that innervate the hindgut of crayfish employ serotonin as a neurotransmitter. Instead of synthesizing serotonin, however, these hindgut neurons (HGNs) appear to "borrow" serotonin by taking it up from external sources. These uptake mechanisms appear to be under the control of exteroceptive and proprioceptive sensory input. The overall goal of this research is to investigate the mechanisms that underlie this "borrowed transmitter" phenomenon in craylish, and to determine how it contributes to the control of serotonergic modulation in the crayfish. Specifically, the proposed research will determine quantitatively whether serotonin is preferentially borrowed or synthesized by the HGNs. The mechanisms that govern borrowing, including those that controi the supply of serotonin made available in the vicinity of the HGNs by external sources, and those that control serotonin uptake and release by the HGNs, will be investigated. The use to which serotonin is put by the HGNs will be determined, as will the effects of serotonin uptake on other adjacent and well-studied serotonergic systems, including those that modulate posture and escape.